The present invention covers a method and a device for reducing the power of jamming signals received by the secondary lobes of a random frequency radar antenna.
In general, these signals are active, continuous or discontinuous jamming signals emitted by several independant jammers. They are received by the radar antenna secondary lobes and their level is such that they are considered as real signals coming from targets and they thus completely upset the radar operation.
In a fixed frequency radar, to guard against active jamming of this type, the countermeasure technique called SLC, `side lobe cancellation`, has been proposed. A schematic description of this can be found in an article by M. A. JOHNSON and D. C. STONER, `ECCM from the radar designer's viewpoint`, which appeared in the Microwave Journal in March 1978, pages 59 to 60.
For this technique, as well as the radar antenna and its corresponding processing channel, a number N of auxiliary antennas associated with N reception channels are used. A weighted linear combination of the N complex signals delivered by the N auxiliary channels is made and this is subtracted from the main channel signal, the weighting coefficients being determined from signals received by the auxiliary channels and the main channel in order to reduce the resulting jamming power in the main channel.
However, the effectiveness of the process as given above, which can be applied to a random frequency radar, appears to be based on the determination of the relative gains of the jamming signals in each of the auxiliary channels with respect to the gain of the main channel. Under these conditions it is then found that any signal other than the jamming signals properly speaking, i.e. thermal noise, clutter, useful echoes, must be considered as a jamming signal upsetting the exact determination of the weighting coefficients and, because of this, interfering with the quality of the jamming elimination. As a result, in devices made under previous practice, the measurement of the weighting coefficients is so made that no account is taken of the existence, in the range sections or range slots divided up in the radar repetition periods, of the effective presence of jamming on the one hand and of that of the jamming signals which have been defined on the other. Hence, the weighting coefficients are computed in a not very precise fashion.
The first way to combat jamming consists in using random frequency signal transmission, the change in frequency taking place at the rhythm of 1/.DELTA.t which is such that the length of fixed frequency recurrence trains is at the most equal to the stationariness of the gains corresponding to the jamming signals received.